Selenium (Se) has been used for the production of solid-state photoelements and current rectifiers. Se, however, has potential application in semiconductor liquid junction solar cells and before the introduction of digital photography it was used in the reproduction of photographs. In the solid state, Se has several allotropic forms. Crystalline allotropes of Se include two monoclinic forms and one hexagonal form. The amorphous and monoclinic forms of Se are dark red to black and red, respectively. The hexagonal form is a semiconductor and it is gray. See, for example, A. K. Graham et al. “Electrodeposition of Amorphous Selenium,” JECS, vol. 106, issue 8, pgs. 651-654 (1959) (hereinafter “Graham”). At room temperature, Se is a p-type semiconductor with a bandgap energy of Eg=1.9 eV exhibiting a direct optical transition with a correspondingly high absorption coefficient.
Recently, there has been a demand for copper indium selenide (CIS), copper indium gallium selenide (CIGS) and copper zinc tin sulfide (CZTS) materials, all of which contain Se. These materials are being used as absorber layers in photovoltaic devices. To form the absorber layer, evaporation methods are typically employed to deposit the various components of the material. Evaporation methods, however, are expensive, deposition rate is very low and a lot of material is lost during the process.
It is known that plating of Se-containing thin films by electrochemical methods (electrodeposition) is extremely difficult due the non-adherent, powdery and resistive nature of Se. There is very little literature on the electroplating of Se films. In a few studies researchers were able to plate, at best, a few monolayers of amorphous and resistive Se. See, for example, Graham (wherein amorphous Se was electroplated on nickel (Ni) up to a thickness of 30 micrometers (μm)). There is a large body of literature on Se being plated as an alloy with copper, indium and gallium (CIS/CIGS-alloy), but these studies do not apply to electroplating only Se.
Therefore, improved techniques for electrodepositing Se-containing films would be desirable.